1. Field of the Invention
This invention relates to a method of forming an electric welded steel tube, wherein a hot-rolled sheet is formed into a cylindrical shape, with the central portion thereof being lowered as the forming progresses, and thereafter, subjected to reduction in the circumferential direction of the tube by means of tandem type fin-pass rolls to be finished into the tube, and more particularly to a method of forming an electric welded steel tube, being suitable for use in a process of forming an electric welded steel tube, in which cage rolls are used to form a tube, and capable of preventing occurrence of edge waves in the tube seam edge portion and/or of cambers in the longitudinal direction of the tube.
2. Description of the Prior Art
In general, an electric welded steel tube is produced by means of cage rolls as follows. More specifically, as shown in FIGS. 1 and 2, a hot-rolled sheet 10 is progressively formed into a cylindrical shape by means of breakdown rolls 12, edge forming rolls 14, outside cage rolls 16 and inside cage rolls 18 in the initial and middle stages, and thereafter, subjected to reduction in the circumferential direction of the tube by means of tandem type fin-pass rolls 20, 22, 24, being the finishing rolls and comprising: top rolls 20a, 22a and 24a; side rolls 20b, 22b and 24b and bottom rolls 20c, 22c and 24c, and finished into a tube 26 having a predetermined dimension of the tubular shape, with special care being paid to a stable forming of an edge portion 10a. FIG. 3 shows the outline of the finished state of the tube in the first fin-pass rolls 20. The tube 26, which has been subjected to reduction in the circumferential direction of the tube, is subjected to high frequency heating at both edge portions 26a of the seam thereof, and upset-welded by means of squeeze rolls 28 comprising top rolls 28a, side rolls 28b and bottom rolls 28c to be formed into an electric welded steel tube 29. Additionally, in this cage roll forming, during the initial and middle stages of the forming in general, as shown in FIGS. 2, 4(A) and 4(B) a so-called downhill forming is practiced in which the central portion 10b of the hot-rolled sheet 10 is lowered to a base line BL as the forming progresses, whereby a difference between the lengths of paths followed by the edge portion 10a and the central portion 10b of the hot-rolled sheet 10 is minimized, to thereby control the longitudinal elongation of the edge portion 10a. Further, the edge portion 10a is continuously, restrainedly supported by means of a plurality of outside cage rolls 16 arranged continuously, whereby a smooth bending occurs.
The downhill type cage roll forming features few occurrences of the edge wave 10c during the initial and middle stage of the forming as compared with the conventional step roll forming in which the hot-rolled sheet 10 is formed into a tube 26 by use of breakdown rolls 30 and side cluster roll 32 and fin-pass rolls 34 as shown in FIG. 5. However, with this cage roll forming, during the last stage of the forming, i.e., the zone of the fin-pass forming corresponding to the finishing step, there have been some cases where a longitudinal compressive force acts on the sheet edge portion 10a, which has been extended during the initial and middle stage of the forming, and, when this compressive force exceeds the buckling stress limit of the sheet edge portion 10a, edge waves have occurred. In general, the formed state of the tube edge portion exerts a considerable influence to the quality of the welded portion in shape, and hence, in particular, there have been encountered with such serious problems as deteriorated quality of the welded portion in shape caused by the edge wave, decreased yield in material and lowered productivity.
Then, in the cage roll forming as described above, a combination of a downhill value D.sub.H of the hot-rolled sheet 10, a total reduction R by the tandem type fin-pass rolls, distribution of the reduction and the like constitutes one of the significant conditions of the forming. However, this combination is not determined definitely, but there are numerous combinations, and the fact is that, heretofore, various conditions for the forming have been empirically adopted. However, the quantitative grasp has not been satisfactorily attained, difficulties have been felt in selecting the proper combination of the conditions of the forming, there have still been occurring edge waves due to mistaken selection of the conditions of the forming in the actual operation, and, particularly, when a tube of non-experience size is produced, difficulties have been encountered in selecting the conditions for the forming and there has been a tendency that occurrence of edge waves has been high in frequency.
In the method of forming a tube as described above, depending upon the selected downhill conditions in the aforesaid forming zone and the selected fin-pass forming conditions, there have been the disadvantage that a camber occurred in the longitudinal direction of the tube 26 after the fin-pass forming as shown in FIG. 6(A) or 6(B). Referring to the drawing, designated at S is a seam portion. Heretofore, this camber of the tube has been sized and corrected by sizing rolls in one of the later processes. However, selection of the conditions of setting the sizing rolls for the sizing and correcting has been very difficult because these rolls are the rolls for the final forming to determine the accuracies in shape and dimensions of the tubular product. As has been described above, heretofore, there has not been performed control in the forming for preventing a camber in the longitudinal direction of the tube by selecting the fin-pass forming conditions, downhill conditions and the like, and the camber caused to the tube has been corrected by sizing rolls in one of the later processes, thus presenting the serious problems including lowered productivity due to increased working time for the correction and decreased accuracies in dimensions of shape through unsatisfactory correction.